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Walgreens Software Engineer Interview Guide

Walgreens Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

Walgreens hires Software Engineers; this guide collects what candidates report about the process.

This guide is scoped to a Software Engineer candidate at Walgreens.

Focus areas

  • Behavioral Interviews
  • Systems Design
  • Scalability

The Walgreens interview process

Walgreens candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Application Review

Initial outreach or application submission to begin the interview process.

What to demonstrate

  • Initial outreach or application submission to begin the interview process
  • Depth in Behavioral Interviews

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Logistical Screening

Screening conducted by an internal recruiter or third-party agency focusing on logistics, citizenship, and compensation.

What to demonstrate

  • Screening conducted by an internal recruiter or third-party agency focusing on logistics, citizenship, and compensation
  • Depth in Behavioral Interviews

How to prepare

  • Answer aloud and timed: Explain the difference between abstract classes and interfaces, and when you would use each in a real-world project.
  • Answer aloud and timed: What programming languages are you most familiar with, and how do they handle memory management?

Technical Phone Interview

Interview with a team lead or senior engineer that includes a resume walkthrough and a live coding exercise.

What to demonstrate

  • Interview with a team lead or senior engineer that includes a resume walkthrough and a live coding exercise
  • Depth in Behavioral Interviews

How to prepare

  • Answer aloud and timed: Walk through how you would implement a basic search algorithm over a localized dataset.
  • Answer aloud and timed: Design a distributed inventory management system that syncs stock levels across 9,000 physical stores in real time.

Multi-Round Interview Loop

Comprehensive assessment consisting of coding, system design, behavioral, and troubleshooting interviews.

What to demonstrate

  • Comprehensive assessment consisting of coding, system design, behavioral, and troubleshooting interviews
  • Depth in Behavioral Interviews

How to prepare

  • Answer aloud and timed: How would you design a highly available notification service to alert patients when their prescriptions are ready for pickup?
  • Answer aloud and timed: Explain how you would handle network latency and offline capabilities for software running inside physical retail stores.

Where candidates lose points

Master the Troubleshooting Mindset

Walgreens values engineers who can keep cool under pressure. Practice explaining your debugging process step-by-step. Start with high-level systems (CDN, load balancer) and work your way down to the database and network layers.

Expect the Unexpected in Scheduling

Because Walgreens utilizes a mix of internal recruiters, third-party agencies, and automated tools, the scheduling process can sometimes feel disjointed. Remain patient but persistent.

Writing code before the input contract is pinned down

Before the first line, state the types, the size bounds, whether duplicates, negatives or an empty input are possible, whether the input is sorted, whether you may mutate it, and what the function returns when nothing matches. Every one of those answers changes the code, and discovering one at minute twenty costs a rewrite you no longer have time for.

Questions & practice

Write a function to reverse a string or array in place, optimizing for memory usage.

Medium · Coding & Problem Solving

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Given an array of integers, find all unique pairs that sum up to a specific target value.

Medium · Coding & Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Explain the difference between abstract classes and interfaces, and when you would use each in a real-world pr

Medium · Coding & Problem Solving

Explain the difference between abstract classes and interfaces, and when you would use each in a real-world project.

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

What programming languages are you most familiar with, and how do they handle memory management?

Medium · Coding & Problem Solving

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Walk through how you would implement a basic search algorithm over a localized dataset.

Medium · Coding & Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

  1. One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
  2. Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
  3. State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
  4. Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

  • A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
  • Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

Explain why the owner filter ignores the listing index

Medium · composite index · query plans · sort elimination

The only index on resource is (tenant_id, status, updated_at DESC, resource_id DESC). A new endpoint returns one user's resources across all statuses, newest created first: WHERE tenant_id = $1 AND owner_user_id = $2 ORDER BY created_at DESC LIMIT 20. On a tenant with 2M rows it takes 900 ms and EXPLAIN shows a sort above a large scan. Explain precisely why the existing index cannot serve it, give the index that can, and state which of these the new index still will not help: owner_user_id alone across tenants; the same query ordered by updated_at. PostgreSQL 16.

Approach

  1. Separate the two jobs an index does. For filtering, a composite btree is seekable only on a left prefix, so with no predicate on status the scan can at best range over tenant_id and test owner_user_id per row; PostgreSQL 16 has no btree skip scan to jump the unconstrained column.
  2. For ordering, the index is sorted by (status, updated_at) within a tenant and not by created_at, so the LIMIT cannot stop early: every matching row is read and then sorted. That is the 'Sort Method: top-N heapsort' line, and it is why the plan reads 2M rows to answer with 20.
  3. Derive the replacement from the access path — equality, equality, then the ordering column: CREATE INDEX CONCURRENTLY ON resource (tenant_id, owner_user_id, created_at DESC). The scan seeks to the (tenant, owner) range and walks 20 entries in order, so the Sort node disappears along with the row-read.
  4. Treat INCLUDE (title, status) as conditional, not free. An index-only scan still visits the heap for any row whose page is not marked all-visible, so on a table taking 1.2k writes/second the win depends on autovacuum keeping the visibility map current, and the wider index costs more on every insert.

Follow-up

  • 90% of rows are status='active'. Would a partial index WHERE status = 'active' change your answer, and for which of the three queries?
  • A dashboard runs this for 40 owners in one page load. What changes about the design?

Design a distributed inventory management system that syncs stock levels across 9,000 physical stores in real

Medium · System Design & Architecture

Design a distributed inventory management system that syncs stock levels across 9,000 physical stores in real time.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

How would you design a highly available notification service to alert patients when their prescriptions are re

Medium · System Design & Architecture

How would you design a highly available notification service to alert patients when their prescriptions are ready for pickup?

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Explain how you would handle network latency and offline capabilities for software running inside physical ret

Medium · System Design & Architecture

Explain how you would handle network latency and offline capabilities for software running inside physical retail stores.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Describe your approach to migrating a legacy monolithic application to a microservices-based cloud architectur

Medium · System Design & Architecture

Describe your approach to migrating a legacy monolithic application to a microservices-based cloud architecture.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

How do you ensure data security and HIPAA compliance when designing systems that handle sensitive patient info

Medium · System Design & Architecture

How do you ensure data security and HIPAA compliance when designing systems that handle sensitive patient information?

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

How do you identify and mitigate a performance bottleneck in a high-traffic API endpoint during peak shopping

Medium · Troubleshooting & Production Support

How do you identify and mitigate a performance bottleneck in a high-traffic API endpoint during peak shopping hours?

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

A critical database synchronization service fails, causing inventory discrepancies across multiple stores. Wha

Medium · Troubleshooting & Production Support

A critical database synchronization service fails, causing inventory discrepancies across multiple stores. What are your immediate steps to diagnose and resolve the issue?

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Describe a time when you had to debug an issue in production without access to local debugging tools. What log

Medium · Troubleshooting & Production Support

Describe a time when you had to debug an issue in production without access to local debugging tools. What logs or metrics did you rely on?

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

How do you design systems to fail gracefully so that a local store outage does not crash the entire regional n

Medium · Troubleshooting & Production Support

How do you design systems to fail gracefully so that a local store outage does not crash the entire regional network?

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Walgreens candidates report.

Day 1 — Map the Walgreens loop

  • Write out the reported sequence: Application Review, Logistical Screening, Technical Phone Interview, Multi-Round Interview Loop.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.

Day 2 — Work Behavioral Interviews

  • Spend the session on Behavioral Interviews, which Walgreens candidates report being tested on.
  • Write one worked example in Behavioral Interviews and time yourself on it.

Deliverable: One timed worked example in Behavioral Interviews.

Day 3 — Work Systems Design

  • Spend the session on Systems Design, which Walgreens candidates report being tested on.
  • Write one worked example in Systems Design and time yourself on it.

Deliverable: One timed worked example in Systems Design.

Day 4 — Work Scalability

  • Spend the session on Scalability, which Walgreens candidates report being tested on.
  • Write one worked example in Scalability and time yourself on it.

Deliverable: One timed worked example in Scalability.

Day 5 — Answer out loud: Coding & Problem Solving

  • Answer aloud, timed: Write a function to reverse a string or array in place, optimizing for memory usage.
  • Answer aloud, timed: Given an array of integers, find all unique pairs that sum up to a specific target value.

Deliverable: Spoken answers to 2 reported Coding & Problem Solving question(s), under time.

Day 6 — Answer out loud: System Design & Architecture

  • Answer aloud, timed: Design a distributed inventory management system that syncs stock levels across 9,000 physical stores in real time.
  • Answer aloud, timed: How would you design a highly available notification service to alert patients when their prescriptions are ready for pickup?

Deliverable: Spoken answers to 2 reported System Design & Architecture question(s), under time.

Day 7 — Answer out loud: Troubleshooting & Production Support

  • Answer aloud, timed: A critical database synchronization service fails, causing inventory discrepancies across multiple stores. What are your immediate steps to diagnose and resolve the issue?
  • Answer aloud, timed: How do you identify and mitigate a performance bottleneck in a high-traffic API endpoint during peak shopping hours?

Deliverable: Spoken answers to 2 reported Troubleshooting & Production Support question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Walk me through a challenging technical project from your past experience. What was your role, and what was th

Medium · Behavioral & Experience

Walk me through a challenging technical project from your past experience. What was your role, and what was the outcome?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a situation where you had a disagreement with a technical lead or product manager. How did you resolv

Medium · Behavioral & Experience

Describe a situation where you had a disagreement with a technical lead or product manager. How did you resolve it?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you prioritize tasks and manage your time when working under tight deadlines or sudden production emerg

Medium · Behavioral & Experience

How do you prioritize tasks and manage your time when working under tight deadlines or sudden production emergencies?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Tell me about a time you had to learn a new technology quickly to deliver a project. What was your approach?

Medium · Behavioral & Experience

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?